How this calculator works
The equation relates bicarbonate to dissolved CO2 at approximately 37 degrees Celsius. Enter bicarbonate and PCO2 from the same venous sample. Blood-gas analysers often calculate bicarbonate from measured pH and PCO2, so reversing that calculation does not provide an independent check. The result is an estimate for information only and is not medical advice.
Formula
pH = 6.1 + log10([HCO3] / (0.03 × PCO2 in mmHg)). kPa is converted to mmHg before calculation.
Worked example
Bicarbonate 24 mmol/L and PCO2 40 mmHg give 6.1 + log10(20) = 7.401. With 12 mmol/L and 40 mmHg, pH is 7.100.
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Frequently asked questions
Can bicarbonate be zero?
No. Both bicarbonate and CO2 pressure must be positive because the equation takes a logarithm.
Can this convert venous results to arterial results?
No. It calculates the pH of the entered sample. There is no fixed offset that makes venous and arterial results interchangeable.
Why might the measured pH differ?
Temperature correction, sampling, rounded constants and measurements made at different times can all affect agreement.